br Funding This work was

Funding This work was partially supported by grants from: ERC-2013 AdG MUNCODD GA340172, AriSLA full grant 2014ARCI, Epigen Flagship Project EPIGENOMICS, AFM TelethonGA17835, Fondazione Roma and Parent Project to I.B.
Conflict of interest
Author contributions
Acknowledgments
Introduction Acute muscle damage triggers the activation and expansion of Pax7+ tissue-specific stem RVX-208 called satellite cells (myogenic progenitors, MPs). Satellite cell proliferation gives rise to a population of CD31−:CD45−:Sca1−:α7 integrin+ myoblasts, that embark into a stepwise process characterized by the sequential upregulation of myogenic regulators such as MyoD, myogenin and MRF4 to eventually lead to differentiation (Le Grand and Rudnicki, 2007). Upon differentiation, myoblasts fuse and rebuild the damaged myofibers, regenerating the architecture of the muscle. Acute skeletal muscle damage also triggers the activation of a population of CD45−:CD31−:α7− integrin:Sca1+ mesenchymal progenitors that reside in the interstitial space between muscle fibers. Based on their ability to originate adipocytes and fibroblasts both in vivo and in vitro, these cells have been named fibro/adipogenic progenitors (FAPs) (Joe et al., 2010; Uezumi et al., 2010; Heredia et al., 2013). FAPs proliferate early during the response to acute damage, and they transiently synthesize extracellular matrix (ECM). Such fibrogenic stage is brief and its end is marked by both a decline in the number of FAPs and clearance of the collagen deposited in the extracellular space (Joe et al., 2010; Uezumi et al., 2010; Lemos et al., 2015). Recent data indicate that besides their fibrogenic activity, FAPs support developmental (Mathew et al., 2011) and regenerative myogenesis through the release of promyogenic cytokines, including IL6 and IL10 (Joe et al., 2010; Lemos et al., 2012). In order to confirm their relevance in muscle regeneration, experiments involving genetic ablation of FAPs have been attempted, reporting somewhat detrimental effects to the regenerative process (Murphy et al., 2011). However, these studies have been somewhat limited by the inability to fully ablate FAPs in skeletal muscle. An alternative strategy is to pharmacologically inhibit FAP expansion following acute damage. Tyrosine kinase inhibitors (TKIs) have been effectively used in the treatment of the most common and progressive forms of human fibrosis (Rosenbloom and Jiménez, 2008; Beyer and Distler, 2013). Nilotinib (Tasigna®, AMN107; Novartis), a second-generation TKI, has been rationally designed to overcome Imatinib resistance in Chronic Myeloid Leukemia (Saglio et al., 2010) and is characterized by better bioavailability, tolerability and lacks the toxic effects commonly seen with Imatinib treatment, such as fluid retention, edema, and weight gain (Kantarjian et al., 2006). Nilotinib showed a more potent antifibrotic effect than Imatinib in liver and lung fibrosis (Rhee et al., 2011; H. Liu et al., 2011; Y. Liu et al., 2011; Shaker et al., 2011). Nilotinib simultaneously targets PDGFR and TGFβ pathways, which explains its potent antifibrotic effects. These data suggest that TKI, could be effectively used as antifibrotic agents (Rokosz et al., 2008). Taking advantage of the fact that both PDGFR –a tyrosine kinase family member receptor- and TGFβ receptor drive FAP activity in vitro and in vivo (Uezumi et al., 2011; Lemos et al., 2015), here we used Nilotinib to pharmacologically block FAPs in the context of skeletal muscle regeneration. Our results show that Nilotinib reduces FAP proliferation and expansion, dampening transient fibrogenesis during muscle regeneration, an effect that is associated with a reduction of fibrogenic gene expression and collagen deposition. We also show that this effect correlates with poor regeneration after acute muscle damage, due to non-cell autonomous reduced myoblast expansion. These results suggest that while tyrosine kinase inhibitor-based therapies could prove useful to reduce excessive fibrosis associated with degenerative pathologies, such therapies could also have a detrimental effect on the overall regenerative capacity of healthy patients.